1. Field of the Invention
The present invention relates to an electroluminescence (EL) element, and a method of driving a light emitting device manufactured by forming thin-film transistors (hereinafter abbreviated as TFTs) on a substrate. Particularly, the present invention relates to an electronic apparatus which uses the light emitting device as display units. Here, as a typical example of light emitting elements, the EL elements will be described.
In this specification, the EL elements include the ones which utilize emission of light from singlet excitons (fluorescence) and the ones which utilize the emission of light from triplet excitons (phosphorescence).
2. Description of the Related Art
In recent years, light-emitting devices having EL elements as the light emitting elements have been vigorously developed as light emitting elements. Unlike liquid crystal display devices, the light-emitting device is of self emission type. The EL element has a structure in which an EL layer is held between a pair of electrodes (anode and cathode), the EL layer being, usually, of a laminated-layer structure. Typically, there can be exemplified a laminated-layer structure of “positive hole-transporting layer/light-emitting layer/electron-transporting layer” proposed by Tang et al. of Eastman Kodak Co. This structure features a very high light-emitting efficiency, and the EL display devices that have now been studied and developed have almost all been employing this structure.
There can be further exemplified a structure in which a positive hole-injecting layer, a positive hole-transporting layer, a light-emitting layer and an electron-transporting layer are laminated in this order on the anode, or a structure in which the positive hole-injecting layer, the positive hole-transporting layer, the light-emitting layer, the electron-transporting layer and the electron-injecting layer are laminated thereon this order. The light-emitting layer may further be doped with a fluorescent pigment or the like pigment.
Here, the layers provided between the cathode and the anode are all referred generally as an EL layer. Therefore, the above positive hole-injecting layer, positive hole-transporting layer, light-emitting layer, electron-transporting layer and electron-injection layer are all included in the EL layer.
A predetermined voltage is applied across the pair of electrodes (both electrodes) holding the EL layer of the above structure therein, whereby the carriers are recombined in the light-emitting layer to thereby emit light. At this time, the light emitting brightness of the EL element is proportional to a current flowing to the EL element.
Brightness changes in a light emitting device that uses EL elements due to deterioration of the elements themselves, even if a fixed current flows. If this kind of deterioration develops and the EL elements are used as a light emitting device, then display pattern burn-in develops, and correct gray scale display becomes impossible to perform.
In particular, brightness changes due to deterioration of the EL elements themselves during their initial switch-on period, referred to as “initial deterioration”, are remarkable. A method of applying a reverse bias to the EL elements in order to suppress deterioration of the EL elements themselves has been proposed in JP 2001-109432 A, JP 2001-222255 A, and the like. A state in which a voltage is applied between an anode and a cathode so that electric current flows in an EL element, namely a state in which the electric potential of the anode is higher than the electric potential of the cathode, is taken as a forward bias here. Conversely, a state in which the electric potential of the cathode is higher than the electric potential of the anode is taken as a reverse bias. If a forward bias is applied, electric current flows in the EL element according to the voltage and the EL element emits light. Electric current does not flow in the EL element and the EL element does not emit light, if a reverse bias is applied.
In addition, a driving method in which a bias applied to an EL element is periodically switched between a forward bias and a reverse bias is defined as alternating current drive here.
Passive matrix and active matrix types exist as light emitting device types, and active matrix light emitting devices are suitable for displays that demand high speed operation because of an increase in the number of pixels that accompanies higher resolution, or in order to perform dynamic display.
Further, a digital time gray scale method, which is not easily influenced by dispersion in the characteristics of driver TFTs, is available as a method of driving an active matrix light emitting device.
In addition, a high precision, multi-gray scale display can be achieved by using an erasure TFT, in addition to a driver TFT and a switching TFT, within each pixel with a digital time gray scale method, as disclosed by JP 2001-343933 A. This driving method is hereinafter referred to as SES (simultaneous erase scan) drive within this specification.
If the EL elements themselves deteriorate, then a difference in the brightness of each of the pixels develops as discussed above corresponding to the degree of deterioration, a display pattern becomes burned in, and correct gray scale display becomes impossible to perform.
In particular, brightness changes due to deterioration of the EL elements themselves during their initial switch-on period, referred to as “initial deterioration”, are remarkable. A method of applying a reverse bias to the EL elements in order to suppress deterioration of the EL elements themselves has been proposed.